\(f(\text { direct })=f\left(\frac{340}{340-5}\right)=f_{1}\)
\(f\left(b y \text { wall) } f\left(\frac{340}{340+5}\right)=f_{2}\right.\)
Beats \(=\left(f_{1}-f_{2}\right)\)
\(5=f\left(\frac{340}{340-5}-\frac{340}{340+5}\right)\)
\(\Rightarrow \mathrm{f}=170 \mathrm{Hz}\)